Abstract: Long tails and streams of stars are the most noticeable
upshots of galaxy collisions. Their origin as gravitational, tidal,
disturbances has however been recognized only less than fifty years ago
and more than ten years after their first observations. This Review
describes how the idea of galactic tides emerged, in particular thanks
to the advances in numerical simulations, from the first ones that
included tens of particles to the most sophisticated ones with tens of
millions of them and state-of-the-art hydrodynamical
prescriptions. Theoretical aspects pertaining to the formation of tidal
tails are then presented. The third part of the review turns to
observations and underlines the need for collecting deep
multi-wavelength data to tackle the variety of physical processes
exhibited by collisional debris. Tidal tails are not just stellar
structures, but turn out to contain all the components usually found in
galactic disks, in particular atomic / molecular gas and dust. They host
star-forming complexes and are able to form star-clusters or even
second-generation dwarf galaxies. The final part of the review
discusses what tidal tails can tell us (or not) about the structure and
content of present-day galaxies, including their dark components, and
explains how tidal tails may be used to probe the past evolution of
galaxies and their mass assembly history. On-going deep wide-field
surveys disclose many new low-surface brightness structures in the
nearby Universe, offering great opportunities for attempting galactic
archeology with tidal tails.